Standard thermal imaging systems of nearly all makes, types and varieties typically employ arrays of planar pixels arranged in a focal plane array (FPA) to detect thermal electromagnetic radiation (EMR) of the image of some distant scene created by imaging optics, in the manner that a typical digital camera does with its CMOS detector array. FIG. 11 illustrates a schematic of such an imaging system 1000 of such a camera with a lens 1020 as the imaging optics, imaging EMR from an object 1050 onto a real image 1060 on a FPA 1010. In FIG. 11, the lens has a focal length f, an object distance S1 and an image distance S2 as shown.
The depth of field of such a camera, i.e., the range of distances in focus at the FPA 1010, can be quite small for close up objects, where S1 would typically be centimeters, to a few meters. As S1 increases from a few meters distance, however, the depth of field increases to a large distance. There is little or no way effectively to reduce that great depth of field except by reducing the f/number (focal length/diameter) of the imaging lens. Typically the f/number will only be increase so far, up to ˜f/1, and at considerable cost in a thermal imager system.
Thus, conventional thermal imaging systems which employ a planar FPA and image a distant object may suffer from issues which occur due to a large depth of field, and range discrimination of an object may be problematic.
One object of thermal imaging where an object must be imaged at a large distance is clear air turbulence (CAT). CAT is the turbulent movement of air masses in the absence of any visual cues such as clouds, and is caused when bodies of air moving at widely different speeds meet. The atmospheric region most susceptible to CAT is the high troposphere at altitudes of around 7,000-12,000 meters (23,000-39,000 ft) as it meets the tropopause. Here CAT is most frequently encountered in the regions of jet streams. At lower altitudes it may also occur near mountain ranges. Thin cirrus clouds can also indicate a high probability of CAT.
CAT can be hazardous to the comfort, and even safety, of air travel. The thermal characteristics of CAT are known. Studies show that gust velocity changes in CAT of at least 20 ft sec−1 are associated with temperature changes of 3° C. or higher; very few being less than 1° C. Such studies show that CAT horizontal temperature gradients with a minimum temperature change of 2° C., and at a rate which equaled or exceeded 0.5° C. per minute. Moderately choppy CAT was observed at a 5° C. temperature change.
Conventionally, CAT has been measured using active electro-optical heterodyne laser velocimeter systems at ranges exceeding 10 km. Such active systems typically use 10 micron wavelength LWIR (long wavelength infrared) CO2 lasers, larger germanium optics and heterodyning optics. Fast, complex signal and data processing renders systems constructed along these line are expensive, power-hungry, heavy, and physically large. Further such active systems require much maintenance on a use-by-use basis in alignment, cleaning etc.